Extruded Structure With Equilibrium Shape

ABSTRACT

An extrusion head is disposed over a substrate, and material is extruded through an oblique (e.g., semi-circular or tapered) outlet orifice of the extrusion head to form an associated extruded structure having an equilibrium shape that resists settling after being deposited on the substrate. The extrusion head includes fluidic channels having a flat surface formed by a flat first (e.g., metal) sheet, and an oblique (e.g., substantially semi-cylindrical) surface formed by elongated oblique trenches that are etched or otherwise formed in a second sheet. The fluidic channel communicates with the outlet orifice, which has a flat edge formed by the first sheet, and an oblique edge formed by an end of the oblique trench. The material is extruded through the outlet orifice such that its flat lower surface contacts the substrate, and its oblique upper surface faces away from the substrate. Two materials are co-extruded to form high aspect-ratio gridlines.

FIELD OF THE INVENTION

The present invention is related to extrusion systems and methods, andmore particularly to micro extrusion systems and methods forco-extruding multiple similar and/or dissimilar materials to formrelatively fine structures with relatively high aspect ratios.

BACKGROUND

With traditional extrusion a billet of material is pushed and/or drawnthrough a die to create a rod, rail, pipe, etc. Various applicationsleverage this capability. For instance, extrusion can be used with foodprocessing applications to create pasta, cereal, snacks, etc., pipepastry filling (e.g., meringue), pattern cookie dough on a cookie pan,generate pastry flowers and borders on cakes, etc. In anotherapplication, extrusion can be used with consumer goods, for example, tomerge different colored toothpastes together on a toothbrush.

FIG. 11 is a perspective view showing an extrusion head 30 of aconventional micro extrusion system for producing fine featured (e.g.,less than 50 micron width and height) structures 20 on the upper surface102 of a substrate 101. Extrusion head 30 that includes metal plates 31,32 and 33 that are laminated together using known high pressure waferbonding techniques, with one or more of the plates being processed todefine a fluidic channel 34 that communicates with an outlet orifice 35that is defined on a side edge of the head. Extrusion material isinserted into fluidic channels 34 through an input port 37 such that theextrusion materials are shaped and extruded through outlet orifice 35,from which they are dispensed onto a target structure (e.g., uppersurface 102 of substrate 101).

FIGS. 12(A) and 12(B) are cross sectional side views illustrating atypical production problem associated with conventional micro extrusionsystems. FIG. 12(A) shows an idealized high aspect-ratio extrudedstructure 20A formed on substrate 101 using the conventional microextrusion techniques described above, with idealized extruded structure20A having the square or rectangular shape of outlet orifice 35. Forpurposes of explanation, idealized extruded structure 20A that has arelatively narrow width W1 and a relatively large height H. A problemwith the production of micro extrusion structures is that the extrudedmaterial is necessarily a fluid (i.e., liquid or paste), and as such issubjected to settling after being extruded. Therefore, the idealrectangular shape shown in FIG. 12(A) typically settles due to itscharacteristics as a fluid, as indicated by the arrows shown in FIG.12(B), causing the idealized high aspect-ratio gridline structure 20B toassume a slumped shape having at least one of a wider width W2 and areduced height H2. This reduction in height and increase in width isundesirable in, for example, solar cell production where the extrudedstructure can be used to form metal gridlines because the settledstructure allows less sunlight to enter substrate 101, and more sunlight(depicted by dashed-line arrows) is reflected away from substrate 101.Consequently, conventional micro extrusion techniques are limited, forexample, in that they cannot render relatively high aspect-ratio (e.g.,1:1 or greater) or porous structures for a cost below $1/sq. ft. Thus,extrusion typically is not used for creating conducting contacts and/orchannels for electrochemical (e.g., fuel), solar, and/or other types ofcells, which leverage high aspect-ratio fine featured porous structuresto increase efficiency and electrical power generation.

Another practical device that benefits from rapid and economical meansfor generating high aspect ratio lines and features include plasmadisplay panels, such as that shown in FIG. 13, where high aspect-ratiobarrier ribs define the sub-pixels within the display. The barrier ribis an electrically insulating structure, and is preferably a high aspectratio structure, as this improves the dot per inch resolution and fillfactor of the display. The settling problem discussed above withreference to FIG. 12(B) results in non-optimal barrier ribs that produceinferior display devices.

What is needed is a system and method for efficiently producing microextrusion structures that can be used, for example, in the production ofhigh quality photovoltaic cells and plasma display panels.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and a method forforming high-aspect ratio functional structures (e.g., “gridlines”) on asubstrate surface in which the gridlines are extruded through an orificeof an extrusion head, wherein the orifice has an oblique (e.g., curvedor tapered) upper surface that causes the gridlines to have a curved ortapered upper surface immediately upon extrusion. The extrusion head isfabricated using several (e.g., metal) sheets that are bonded orotherwise laminated together. One of the sheets is etched to define theoblique surface of the orifice, and that sheet is then bonded to asecond sheet to provide a flat lower surface of the orifice. Inaccordance with the present invention, the oblique upper surface of theorifice is formed such that the gridlines are substantially inequilibrium immediately after being extruded, thus preventingundesirable subsequent settling that increases the width and reduces theheight.

In accordance with an embodiment of the present invention, a gridline(functional) material is co-extruded with a support (e.g., sacrificial)material onto the substrate surface such that the high-aspect ratiogridline is supported between two support material portions (in oneembodiment the support portions are treated as sacrificial portions thatare subsequently removed). The formation of such co-extruded structuresrequires the compression of the gridline material between the twosupport material portions, which requires the use of a relatively widethree-channel cavity feeding a relatively narrow outlet orifice in amanner that compresses the gridline material between the two supportmaterial portions. By forming the composite extruded structure with anequilibrium shape, the present invention facilitates the reliableproduction of high aspect-ratio gridlines.

In accordance with an embodiment of the present invention, a method formanufacturing an extrusion head for a micro extrusion apparatus includesetching a first sheet to include an elongated trench having an oblique(e.g., generally semi-cylindrical or tapered) shape. The trench has aclosed end, and extends to a side edge of the sheet. A second sheet isetched to include an inlet port that is positioned to align with theclosed end of the trench when the first and second sheets are bondedtogether. The oblique trench is thus formed in a reliable and economicalmanner, and serves to provide an orifice having an oblique surface thatis defined in a side edge of the extrusion head.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings, where:

FIG. 1 is an assembled perspective view showing a portion of aco-extrusion head of a micro extrusion system according to an embodimentof the present invention;

FIG. 2 is an exploded perspective view showing the portion of theco-extrusion head of FIG. 1;

FIG. 3 is a perspective view showing a micro extrusion apparatusincluding the co-extrusion head of FIG. 1 for concurrently applying twoor more materials on a substrate;

FIGS. 4(A) and 4(B) are cross-sectional side views showing athree-channel cavity defined in the co-extrusion head of FIG. 1;

FIG. 5 is a cross-sectional side view showing an exemplary co-extrudedgridline structure that was generated on a substrate surface by theco-extrusion head of FIG. 4(B);

FIG. 6 is a cross-sectional side view showing a sheet including a firstmask used to form trenches according to an embodiment of the presentinvention;

FIG. 7 is a cross-sectional side view showing an etching process forforming trenches using the first mask shown in FIG. 6;

FIG. 8 is a cross-section side view showing a second mask and secondetching process used to form an inlet opening into a central trench ofthe sheet shown in FIG. 7;

FIG. 9 is a cross-sectional side view showing a portion of an extrusionhead including the sheet formed in FIG. 8;

FIG. 10 illustrates a photovoltaic cell including gridlines formed inaccordance with the present invention;

FIG. 11 is perspective view showing a portion of a conventional microextrusion head;

FIGS. 12(A) and 12(B) are simplified cross-sectional side views showingextruded structures formed by the conventional head shown in FIG. 11;and

FIG. 13 is a simplified cross-sectional side view showing a portion ofan exemplary plasma display panel.

DETAILED DESCRIPTION

FIG. 1 illustrates a portion of an extrusion head 130, which makes uppart of a micro extrusion apparatus 100 for producing an extrudedstructures 120 having an equilibrium shape on a substrate 101 inaccordance with an embodiment of the present invention. Extrusion head130 is operably coupled to one or more sources (not shown) of extrusionmaterials such that the material is extruded from an outlet orifice 135defined in a side edge 139 of extrusion head 130, and is deposited ontothe upper surface 102 of substrate 101 with the desired equilibriumshape that resists settling after extrusion.

FIG. 2 is an exploded perspective view showing extrusion head 130 inadditional detail. In accordance with an embodiment of the presentinvention, extrusion head 130 is made up of multiple sheets (substratesor plates) 210, 220 and 230 that, in one embodiment, are bonded usingknown high pressure wafer bonding techniques to form the substantiallysolid, block-like structure shown in FIG. 1. In one embodiment, sheets210, 220 and 230 are metal plates having a thickness of approximately0.15 mm.

Each of the sheets has opposing sides and a substantially straight sideedge—sheet 210 has opposing first and second surfaces 211-1 and 211-2and a side edge 219, sheet 220 has opposing surfaces 221-1 and 221-2 anda side edge 229, and sheet 230 has opposing surfaces 231-1 and 231-2 anda side edge 239. Sheets 210, 220, and 230 are bonded such that sheet 230is sandwiched between sheet 210 and 220, with sheet 230 being mounted onsheet 220 such that surface 231-1 faces surface 221-2, and sheet 210being mounted on sheet 230 such that (first) surface 211-1 faces(second) surface 231-2. Sheets 210, 220 and 230 are assembled orprocessed (e.g., by one or more of cutting, milling or grinding) suchthat side edges 219, 229 and 239 are aligned to form a edge surface 139of extrusion head 130, as indicated in FIG. 1. A method for fabricatinghead 130 is described in co-owned and co-pending U.S. patent applicationSer. No. ______, entitled “EXTRUSION HEAD WITH PLANARIZED EDGE SURFACE”[Atty Docket No. 20060464Q-US-NP (XCP-074)], which is incorporatedherein by reference in its entirety.

Referring again to FIG. 2, in accordance with an aspect of the presentinvention, sheet 230 is etched or otherwise manufactured to include atleast one of elongated trenches 232, 233 and 234 that are defined insurface 232-2, and extending from an open end (notch) 235 locatedadjacent to side edge 239 to a closed end disposed away from side edge239. Each trench 232, 233 and 234 has a concave oblique (e.g., generallysemi-cylindrical or tapered) surface that is formed in the mannerdescribed below. Specifically, trench 232 includes an oblique surface231-22, trench 233 includes an oblique surface 231-23, and trench 234includes an oblique surface 231-24. When sheet 230 is subsequentlybonded to sheet 210, planar flat portions 231-21 of second surface 232-2abut first surface 211-1 of sheet 210, and each trench 232, 233 and 234combines with opposing flat portions 211-12, 211-13 and 211-14,respectively, of surface 211-1 to form a generally semi-cylindricalfluidic channel (e.g., fluidic channels 132, 133, 134, indicated in FIG.1), with each fluidic channel communicating with an associated outletorifice 135.

In accordance with another aspect of the invention, outlet orifice 135includes a straight edge 136 that is defined by the portion of flatsurface 211-1 located at side edge 219 of sheet 210, and an obliquesecond edge 137 defined by end portions of oblique surfaces 232-22,232-23, and/or 232-24 that are located at side edge 239 of sheet 230.Oblique edge 137 facilitates the production of extruded structureshaving an equilibrium shape in the manner described below.

In accordance with an embodiment of the present invention, trenches 232,233 and 234 are arranged in an arrowhead-shaped pattern such that, whenextrusion head 130 is assembled, a fluidic channel 130-1 is formed as athree-channel cavity having central channel 132 positioned betweenopposing (first and second) side channels 133 and 134, with all threechannels communicating with output port 135. In particular, at theirrespective closed ends, central trench 232 is separated from sidetrenches 233 and 234 by tapered finger-like flat portions 232-211 and232-212, respectively, and trench 232 is closed by an end flat portion232-313, thereby form central channel 132 when sheets 210 and 230 arecombined. Similarly, side trenches 233 and 234 are closed bycorresponding surrounding flat portions of sheet 210 to form opposingside channels 133 and 134. Side channels 133 and 134 are angled towardcentral channel 132, and converge at a point adjacent to notch 235,which cooperates with sheet 210 to form outlet orifice 135. Although thedisclosed embodiment depicts three intercommunicating trenches/channelsarranged in an arrowhead shape, aspects of the present invention applyto any number of trenches/channels (e.g., one single trench/channelcommunicating with outlet orifice 135).

Referring again to FIG. 1, in accordance with another aspect of thepresent invention, extrusion head 130 is moved relative to substrate 101(e.g., in the direction of arrow A) while one or more extrusionmaterials (not shown) are forced through fluidic channels 132, 133 and134 such that the material is extruded from outlet orifice 135 and formsan associated extruded structure 120 on substrate 101. Mechanisms forgenerating the required relative movement between substrate 101 andextrusion head 130 are well known. The extrusion material is forcedthrough inlets located adjacent to the closed ends of each fluidicchannel 132, 133 and 134 using known techniques. Referring to FIG. 2,the inlet ports used to communicate with the fluidic channels are etchor otherwise formed in the various sheets. In particular, sheet 210defines inlet ports (e.g., through holes, slots or channels) 213 and 214that are aligned with the closed ends of trenches 233 and 234,respectively, and sheet 220 defines an inlet port 222-1 that is alignedwith central trench 232. An inlet opening 222-2 is formed inside centraltrench 232 that extends through the thin remaining wall of sheet 230 andaligns with inlet port 222-1 when sheets 210 and 230 are joined. Inletports 213, 214, 222-1 and 222-2 are formed, for example, usingmicro-machining techniques (e.g., photo-chemical machining, pulsed lasermachining, deep reactive ion etching, electro-discharge machining oranisotropic etching).

In accordance with another aspect of the invention, due to the shape offluidic channels 132, 133 and 134 and outlet orifice 135, extrudedstructure 120 (shown in FIG. 1) has an equilibrium shape upon extrusion,thus avoiding the settling problems associated with conventional microextrusion techniques. In particular, extruded structure 120 has a flatlower surface 126 (i.e., the surface in contact with upper surface 102of substrate 101) that is formed by flat edge 136 of outlet orifice 135,and a curved or tapered upper surface 127 that is formed by the obliqueedge 137 of outlet orifice 135 and faces away from substrate 101. Incontrast to the rectangular shaped initial extrusion structure 20A (FIG.12(A)) generated by conventional micro extrusion techniques which issubject to settling, extruded structure 120 is extruded in a shape thatis close to structural equilibrium, thereby resisting settling andfacilitating the production of extruded structures that have arelatively uniform and reliably consistent height and width.

In addition to the laminated metal layer arrangement depicted in FIG. 1,extrusion head 130 can be manufactured a variety of ways. For example,rounded channels can be formed by electroforming metal over resiststructures that have been reflowed above their glass transitiontemperature. Tapered channels can also be formed by electroforming metalover resist structures that are processed using known techniques forcreating a tapered sidewall. In another embodiment, an extrusion headformed in accordance with the present invention can be manufactured bybrazing together layers of etched sheet metal. In yet another instance,the heads can be manufactured by generating structures out ofphoto-definable polymer such as SU8. In still another instance, theheads can be machined or molded out of metal and/or plastic usingconventional manufacturing techniques.

FIG. 3 illustrates micro extrusion apparatus 100A in accordance withanother embodiment of the present invention. Apparatus 100A includes anextrusion device 110 having one or more co-extrusion heads 130-1 and130-2 fixedly mounted thereon, each co-extrusion head 130-1 and 130-2being consistent with extrusion head 130, described above. In thepresent embodiment, extrusion device 110 is coupled to a first source111 containing a support material 112, and a second source 114containing a functional (“gridline”) material 115. Extrusion heads 130-1and 130-2 are operably coupled to sources 111 and 114 such that heads130-1 and 130-2 concurrently apply support material 112 and a gridlinematerial 115 onto the upper surface 102 of a substrate 101. Thematerials are applied through pushing and/or drawing techniques (e.g.,hot and cold) in which the materials are pushed (e.g., squeezed, etc.)and/or drawn (e.g., via a vacuum, etc.) through extrusion device 110and/or co-extrusion heads 130-1 and 130-2, and out outlet orifices 135that are respectively defined in a lower portion of co-extrusion heads130-1 and 130-2.

In one embodiment, co-extrusion heads 130-1 and 130-2 are held byextrusion device 110 such that their respective outlet orifices arearranged in a parallel, spaced-apart arrangement. In particular, the(first) outlet orifices of co-extrusion head 130-1 (e.g., outletorifices 135-11 and 135-12) extending in a first direction X1, and the(second) outlet orifices of the second co-extrusion head 130-2 (e.g.,outlet orifices 135-21 and 135-22) define a second line X2 that isseparated from and parallel to first line X1. As set forth in co-pendingU.S. patent application Ser. No. ______, entitled “CLOSELY SPACED,HIGH-ASPECT EXTRUDED GRIDLINES” [Atty Docket No. 20060464-US-NP(XCP-072)], which is incorporated herein by reference in its entirety,apparatus 100A includes a mechanism (not shown) for moving extrusiondevice 110 (and, hence, co-extrusion heads 130-1 and 130-2) in adirection that is perpendicular to the alignment direction of the outletorifices, and gridline material 115 and support material 112 areco-extruded through outlet orifices 135 in a manner that createsparallel, elongated extruded structures 120A on substrate 101 such thatthe gridline material of each structure 120A forms a high-aspect ratiogridline structure 125, and the support material of each structure 120Aforms associated first and second support material portions 122respectively disposed on opposing sides of the associated high-aspectratio gridline 125. The shape of extruded structures 120A (i.e., theaspect ratio of gridline 125 and the shape of support portions 122) arecontrolled by the shape outlet orifices 135 and the fluidic channelsinside heads 130-1 and 130-2, characteristics of the materials (e.g.,viscosity, etc.), and the extrusion technique (e.g., flow rate,pressure, temperature, etc.) to achieve the equilibrium shape mentionedabove and described in additional detail below. The structure withinheads 130-1 and 130-2 and the shape of outlet orifices 135 is consistentwith that described above with reference to FIGS. 1 and 2. Suitablegridline materials 115 include, but are not limited to, silver, copper,nickel, tin, aluminum, steel, alumina, silicates, glasses, carbon black,polymers and waxes, and suitable support materials 112 include plastic,ceramic, oil, cellulose, latex, polymethylmethacrylate etc.,combinations thereof, and/or variations thereof, including combining theabove with other substances to obtain a desired density, viscosity,texture, color, etc. The outlet orifices of co-extrusion heads 130-1 and130-2 are disposed in a staggered arrangement to simultaneously generateextrusion structures 120A that are closely spaced, thus facilitating theproduction of high aspect-ratio gridlines 125 are formed on substrate101 at a pitch that is not possible using conventional methods. Inanother embodiment of the present invention, a single head may be usedto produce extrusion structures 120A that are spaced relatively farapart.

To limit the tendency for the materials to intermix after extrusion,extruded structures 120A leaving extrusion heads 130-1 and 130-2 can bequenched on substrate 101 by cooling the substrate using, for example, aquenching component 170. Alternately, the ink/paste used in thisapplication may be a hot-melt material, which solidifies at ambienttemperatures, in which case the printheads 130-1 and 130-2 are heated,leaving the extruded structures 120A to solidify once they are dispensedonto the substrate 101. In another technique, the materials can be curedby thermal, optical and/or other means upon exit from extrusion heads130-1 and 130-2. For example, a curing component 180 can be provided tothermally and/or optically cure the materials. If one or both materialsinclude an ultraviolet curing agent, the material can be bound up intosolid form in order to enable further processing without mixing.

FIG. 4(A) shows a portion of co-extrusion head 130-1 including fluidicchannel 130-11 positioned over substrate 101 prior to generation ofmetal gridlines. Co-extrusion head 130-1 is maintained at asubstantially fixed distance D over upper surface 102 of substrate 101during the extrusion process (i.e., while co-extrusion head 130-1 ismoved relative to substrate 101 in the manner described above). Thedistance D between the head 130-1 and the substrate 101 can be based onvarious factors, such as the angle of the dispensing end of the head130-1 with respect to upper surface 102 (e.g., from parallel toperpendicular), in order to increase transfer efficiency, entitydefinition (e.g., width, height, length, diameter, etc), entitycharacteristics (e.g., strength, pliability, etc.), etc. Note thatdistance D must be greater than or equal to the height H (shown in FIG.5) of extruded structure 120-11 in order to facilitate the staggeredextrusion head arrangement shown in FIG. 3.

FIG. 4(B) shows the same portion of co-extrusion head 130-1 at the onsetof the co-extrusion process. As indicated by the white arrows, gridlinematerial 115 is forcibly injected through the first inlet ports 222-1and 222-2 (see FIG. 2) into the closed end of central channel 132, andsupport material 112 is simultaneously forcibly injected through inletports 213 and 214 into side channels 133 and 134, respectively. Asindicated by the dark arrows in FIG. 4(B), the injected materials traveldownward along their respective channels. The gridline and supportmaterials are compressed by the tapered shapes of channels 132, 133 and134. The gridline material is further compressed by the convergingsupport material flowing along side channels 133 and 134 as thematerials approach outlet orifice 135-11. The compressed flow is thenextruded from outlet orifice 135-11 and is deposited on substrate 101 asextruded structure 120A-11 (shown in FIG. 5). Intermixing between thegridline and support materials is minimized by choosing appropriatematerials and viscosities, by appropriately tapering the channels,and/or by maintaining laminar flow conditions.

FIG. 5 is a cross-sectional side view showing an exemplary extrudedstructure 120A-11 produced in accordance with the co-extrusion processdescribed with reference to FIG. 4(B). Extruded structure 120A-11includes a gridline 125-11 disposed between support material portions122-1 and 122-2. Due to the trench shape and converging forces generatedby three-branch fluidic channel 130-11 (FIGS. 4(A) and 4(B)) leading tooutlet orifice 135-11, extruded structure 120A-11 exhibits advantagesover gridlines formed by conventional methods. That is, in addition tohaving a flat lower surface 126 and curved or tapered upper surface 127characteristic of the equilibrium shape described above, extrusion head130-1 facilitates the formation of gridline 125-11 with an aspect ratio(height H to width W) of 2:1 or greater in a single pass, which is notpossible using conventional methods. The width W of gridline 125-11 canbe made narrower (finer) than the smallest minimum design feature ofextruder head 130-11. Due to the equilibrium shape, support materialportions 122-1 and 122-2 reliably retain the high-aspect ratio shape ofgridline 125-11 as long as needed before or during subsequent processingsuch as drying, curing, and/or sintering. As shown on the right side ofFIG. 5, the support portions are then removed, thus providing highaspect-ratio gridline 125-11 with the desired height H and width W. Afurther advantage of support material portions 122-1 and 122-2 is thatthe added material leads to an overall larger outlet orifice 135-11, andhence a lower pressure drop for a given material flow speed. Higherprocess speed is therefore achievable. In addition, the compressing flowcan be manipulated to form metal gridline 125-11 with a taperedcross-section (e.g., with a relatively wide base disposed on substratesurface 102, a relatively narrow upper end, and tapered sides thatextend at an angle relative to surface 102 from the base end to theupper end). This tapered shape facilitates directing photons intosubstrate 101, and reduces the photon blocking (shading) caused by thegridlines, which can improve efficiency and/or generation of electricalpower.

FIGS. 6 and 7 are cross-sectional side views illustrating the formationof elongated trenches 232, 233 and 234 in sheet 230 according to anotherembodiment of the present invention. As shown in FIG. 6, a mask 810 ispatterned over a surface of sheet 230 such that windows 815 exposeelongated regions of sheet 230 corresponding to the desired elongatedtrenches. Next, an etchant 820 is applied over mask 810 such thatetchant 820 enters into windows 815 and isotropically etches sheet 820,thereby forming the desired oblique trenches 232, 233 and 234 (FIG. 7).In one embodiment, sheet 230 is 316L stainless steel having a thicknessof 0.010 inches, and etchant 820 is ferric chloride, which is appliedthrough windows 815 having a width of 0.002 inches. Variousmodifications to the etching process may be used to alter the curved ortapered shape of the elongated trenches, such as using laser abalationinstead of chemical etching, or electroplating the structure after themachining process.

As depicted in FIG. 8, a second mask 830 is patterned over surface231-1, and a second etchant 840 is used to form inlet opening 222-2(FIG. 2). This pattern is preferably applied using a two-sided maskaligner. The second etching step can be performed simultaneously or insequence with the first etching step.

FIG. 9 depicts a portion of the fully-assembled extrusion head 130including first sheet 210 and second sheet 220 disposed on opposingsurfaces of sheet 230. Note that sheet 220 includes opening 222-1(described above), which aligns with inlet opening 222-2 to facilitateinjection of gridline material into central channel 132. Similarly,sheet 210 includes openings 213 and 214 (described above), whichfacilitate injection of gridline material into side channels 133 and134, respectively.

FIG. 10 illustrates an exemplary portion of a photovoltaic cell 300,such as a solar cell, with high-aspect metal gridlines 125 created viaco-extrusion head 130 according to an embodiment of the presentinvention. Photovoltaic cell 300 includes a semiconductor substrate 301with a p-type region 306 and an n-type region 308. One or both of theregions 306 and 308 of substrate 301 is formed from semiconductormaterials such as, for example, Aluminum Arsenide, Aluminum GalliumArsenide, Boron Nitride, Cadmium Sulfide, Cadmium Selenide, CopperIndium Gallium Selenide, Diamond, Gallium Arsenide, Gallium Nitride,Germanium, Indium Phosphide, Silicon, Silicon Carbide, SiliconGermanium, Silicon on insulator, Zinc Sulfide, Zinc Selenide, etc. Alower contact 310 is formed on a lower surface 302 of substrate 301(i.e., at a lower end of p-type region 306). Metal gridlines 125 and oneor more bus bars 320 are formed on an upper surface 304 of substrate 301(i.e., at a lower end of n-type region 308). Contact 310 and bus bars320 can be formed using a metal paste such as a silver based paste or analuminum based paste.

Photovoltaic cell 300 can be interconnected with other photovoltaiccells (not shown) in series and/or parallel, for example, via flat wiresor metal ribbons, and assembled into modules or panels and connected asindicated to a load 340. A sheet of tempered glass (not shown) may belayered over the gridlines 125 and/or a polymer encapsulation (notshown) may be formed over the contact 310. Upper surface 304 may includea textured surface and/or be coated with an antireflection material(e.g., silicon nitride, titanium dioxide, etc.) in order to increase theamount of light absorbed into the cell.

During operation, when photons 350 (indicated by wide arrows) aredirected into substrate 301 through upper surface 304, their energyexcites electron-hole pairs therein, which subsequently freely move. Inparticular, absorption of a photon creates an electric current throughthe p-n junction (depicted by the migrating + and − charges). Electricalcurrent is generated when excited electrons in the n-type region 308travel through gridlines 125, bus bar 320, and the electrodes toexternal load 340 and back through the lower electrode and contact 310to the p-type region 306.

By way of example, a co-extrusion head with the estimated parametersillustrated in Table 1 could be used to dispense the materials to makegridlines 125 on a crystalline silicon solar cell.

TABLE 1 Exemplary head parameters for generating a gridline. SheetThickness 152 microns Gridline Pitch 2.5 mm Head Speed 1 cm/sec PastViscosity 100,000 Cp Head Angle 45 degrees Head Exit Width 304.8 MicronsSilver Width 49.2 microns Silver Line Cross Section 7,500microns{circumflex over ( )}2 Silver Line Aspect Ratio 3.10:1 SilverFlow 0.075 mm{circumflex over ( )}3/sec Head Compression 6.2:1 HeadPressure Drop 2.24 atm

With this design, convergent channels are patterned into a sheet ofmaterial with a thickness of approximately 0.15 mm. The outlet orificesof the head/nozzles are repeated on a pitch of 2.5 mm. At a head/nozzlepressure of approximately 2.24 atmospheres, paste of 1000 poise isejected at a rate of 1 cm/sec. The central stripe of silver isapproximately 50 microns wide with an aspect ratio of 3:1.

Although the present invention has been described with respect tocertain specific embodiments, it will be clear to those skilled in theart that the inventive features of the present invention are applicableto other embodiments as well, all of which are intended to fall withinthe scope of the present invention. For example, in addition to stripedmaterials with a lateral variation, variations of head 130 may be usedto additionally and/or alternatively introduce materials with a verticalvariation, for example, for introducing barrier layers onto thesubstrate. Such vertical variation can be implemented by formingchannels that converge dissimilar materials together in the verticaldirection (in addition to converging in the horizontal direction) withinthe manifold. For instance, with a solar cell application, it may beadvantageous to introduce a metal bi-layer onto the cell surface withone metal making contact to the silicon as a diffusion barrier, and asecond metal on top selected for either lower cost or higherconductance. Further, in addition to metal gridlines, the methods andstructures described herein may be utilized to generate gridlines formedfrom electrically non-conductive materials, such as inorganic glassesthat are used, for example, to produce the barrier rib structuresdescribed with reference to FIG. 13.

Furthermore, although in the examples provided, the side and centralchannels are fed from opposite faces of the extrusion apparatus, it isclear that with the necessary modifications, the side and centralchannels can also be fed from a common side, making it possible toextrude material at a grazing angle to the substrate.

1. A micro extrusion apparatus for producing an extruded structurehaving an equilibrium shape on a substrate, the micro extrusionapparatus comprising: an extrusion head including: a first sheet havinga first surface and a first side edge; a second sheet having a secondsurface and a second side edge, wherein the second sheet is mounted onthe first sheet such that a flat portion of the second surface abuts thefirst surface, and the second side edge is aligned with the first sideedge, whereby the first and second side edges form a edge surface of theextrusion head, and wherein the second surface of the second sheetdefines an elongated trench extending from the second side edge andhaving a concave oblique surface, thereby forming a fluidic channelhaving an outlet orifice including a straight first edge defined by thefirst sheet, and an oblique second edge defined by the elongated trench;and means for moving the extrusion head relative to the substrate whileforcing material through the fluidic channel such that material isextruded from the outlet orifice and forms an associated extrudedstructure on the substrate, wherein the extruded structure has a flatlower surface that is formed by the flat first edge of the orifice andis in contact with the surface, and an upper surface that is formed bythe curved second edge of the outlet orifice and faces away from thesubstrate.
 2. The apparatus of claim 1, wherein said means comprises anextrusion device coupled to a source of the extrusion material, andwherein the extrusion head is fixedly mounted on the extrusion device.3. The apparatus of claim 1, further comprising at least one of meansfor heating the extrusion materials before extrusion, means for coolingthe substrate during extrusion, and means for curing the extrudedmaterial.
 4. The apparatus of claim 1, wherein the fluidic channelcomprises a three-channel cavity including a central channel andopposing first and second side channels, wherein the central channelcomprises the elongated trench, and the first and second side channelscomprise second and third elongated trenches, and wherein the centralchannel and the first and second side channels communicate with theoutlet orifice, and wherein the apparatus further comprises means forinjecting functional material into the central channel of thethree-channel cavity while injecting support material into the first andsecond side channels of said three-channel cavity such that saidfunctional material extruded from the outlet orifice forms an associatedhigh aspect-ratio functional structure of said extruded structure, andsaid support material extruded from the outlet orifice forms associatedfirst and second support material portions respectively disposed onopposing sides of said associated functional structure.
 5. The apparatusof claim 4, wherein the first sheet defines a first and second inletports respectively communicating with the first and second side channelsof the three-channel cavity, and the extrusion head further comprises athird sheet having a third inlet port communicating with the centralchannel of the three-channel cavity, and wherein said means forinjecting the functional material and the support material comprisesmeans for forcing the functional material through the third inlet portinto the central channel while forcing the support material through thefirst and second inlet ports into the first and second side channels,respectively.
 6. The apparatus of claim 5, wherein the second sheetfurther defines an inlet opening disposed in the elongated trenchadjacent to the closed end and extending entirely through the secondsheet, and wherein said means for injecting the functional materialincludes means for forcing the functional material through both thethird inlet port and the inlet opening into the central channel.
 7. Themicro extrusion system according to claim 1, wherein the first andsecond sheets comprise metal plates.
 8. A method for forming an extrudedstructure with an equilibrium shape on a substrate, the methodcomprising: positioning an extrusion head adjacent to a surface of thesubstrate such that an outlet orifice defined by the extrusion head isdisposed over the surface, wherein the extrusion head comprises firstand second sheets defining a fluidic channel therebetween thatcommunicates with the outlet orifice, the first sheet having asubstantially flat surface that defines a flat edge of the outletorifice, and the second sheet defining an elongated oblique trench thatdefines an oblique edge of the outlet orifice; moving the extrusion headrelative to the substrate while forcing material through the fluidicchannel such that the material is extruded from the outlet orifice andforms an associated extruded structure on the substrate, wherein theextrusion head is positioned relative to the substrate such that a flatlower surface of the extruded structure is formed by the flat edge ofthe orifice and is in contact with the surface, and an upper surface ofthe extruded structure is formed by the oblique edge of the outletorifice and faces away from the substrate.
 9. The method of claim 8,wherein providing the extrusion head comprises: etching the first sheetto form said elongated oblique trench; etching the second sheet suchthat the second sheet includes an inlet port passing entirely throughthe second sheet; and bonding the first sheet to the second sheet suchthat the inlet port is aligned with the closed end of the elongatedtrench.
 10. The method of claim 8, wherein providing the extrusion headcomprises fixedly mounting the extrusion head on an extrusion devicecoupled to a source of the extruded material.
 11. The method of claim 8,further comprising at least one of heating the extruded materials priorto extrusion, cooling the substrate during extrusion, and curing theextruded materials.
 12. The method of claim 8, wherein the fluidicchannel comprises a three-channel cavity including a central channel andopposing first and second side channels, wherein each of the centralchannel and the first and second side channels comprise an associatedsaid elongated trench and communicate with the outlet orifice, andwherein forcing the material through the fluidic channel comprisesinjecting functional material into the central channel of thethree-channel cavity while injecting the support material into theopposing first and second side channels of the three-channel cavity suchthat said functional material extruded from the outlet orifice forms anassociated high aspect ratio functional structure of said extrudedstructure, and said support material extruded from the outlet orificeforms associated first and second support material portions respectivelydisposed on opposing sides of said associated functional structure. 13.The method of claim 12, wherein the first sheet defines a first andsecond inlet ports respectively communicating with the first and secondside channels of the three-channel cavity, and the extrusion headfurther comprises a third sheet having a third inlet port communicatingwith the central channel of the three-channel cavity, and whereininjecting the functional material and the support material furthercomprises forcing the functional material through the third inlet portinto the central channel while forcing the support material through thefirst and second inlet ports into the first and second side channels,respectively.
 14. The method of claim 13, wherein the second sheetfurther defines an inlet opening disposed in the elongated trenchadjacent to the closed end and extending entirely through the secondsheet, and wherein injecting the functional material includes forcingthe functional material through both the third inlet port and the inletopening into the central channel.
 15. A method for manufacturing anextrusion head for a micro extrusion apparatus, the method comprising:etching a first sheet such that an elongated trench is defined in asurface of the sheet, the elongated trench having an open end disposedat a side edge of the sheet and an opposing closed end, the elongatedtrench having a generally semi-cylindrical surface; etching a secondsheet such that the second sheet includes an inlet port passing entirelythrough the second sheet; and bonding the first sheet to the secondsheet such that the inlet port is aligned with the closed end of theelongated trench, and such that a flat surface portion of the secondsheet is disposed opposite to the elongated trench between the inletport and the side edge, whereby the elongated trench and the flatportion of the second sheet form a fluidic channel that extends betweenthe inlet port and an outlet orifice located at the side edge, whereinthe outlet orifice has a flat edge formed by a side edge of the secondsheet, and an oblique edge formed by the an end of the elongated trench.16. The method of claim 15, wherein etching the first sheet comprisesone of photochemical machining, pulsed laser machining, deep reactiveion etching, electro-discharge machining and anisotropic etching. 17.The method of claim 15, wherein etching the first sheet comprisesforming three elongated trenches including a central trench and opposingfirst and second side trenches, wherein each of the central trench andthe first and second side trenches extend from the side edge to anassociated opposing closed end.
 18. The method of claim 17, whereinetching the second sheet comprises forming first and second inlet portsdisposed such that the first inlet port is aligned with the closed endof the first side trench and the second inlet port is aligned with theclosed end of the second side trench after said bonding.
 19. The methodof claim 18, wherein etching the second sheet further comprises formingan inlet opening disposed in the central elongated trench adjacent toits closed end and extending entirely through the second sheet, andwherein the method further comprises etching a third sheet such that thethird sheet includes a third inlet port passing entirely through thesecond sheet, and wherein bonding the first sheet to the second sheetfurther comprises bonding the third sheet to a second side of the firstsheet such that the third inlet port is aligned with the inlet opening.